This invention relates to stereo radio receivers and more particularly to such stereo receivers including apparatus effective to decrease objectionable high frequency noise.
Stereo receivers are generally adapted to receive a radio frequency signal, derive from it a stereo composite signal and decode the stereo composite signal into left and right channel audio frequency signals. In the case of an FM stereo RF signal the stereo composite signal generally comprises a sum of channels signal in the audio frequency range of approximately 30 to 15 kilohertz, which is required for monaural compatability, and a difference of channels signal comprising a set of side bands of a suppressed 38 kilohertz subcarrier, the side bands extending from 23 to 53 kilohertz, a frequency range above the audio frequency range.
Such FM stereo receivers are subject to the reproduction of noise along with the desired signals if said noise happens to be in the same frequency range as the signals. This noise may originate externally of the receiver, but it may also originate within the internal receiver circuits. The latter type of noise is especially objectionable in the higher audio frequencies and the frequency range of the difference of channels signal.
Many prior art FM stereo receivers have included apparatus for reducing objectionable noise in the reproduction of a received FM stereo signal. One of this type of receiver has been shown in the U.S. Pat. No. 3,952,161 to W. F. Gilbert et al. This receiver employs standard FM stereo receiver circuitry and methods for processing the received signal through the stereo decoder, where left and right channel audio signals are derived. These left and right channel signals are processed, however, by an audio processor which includes a high frequency roll-off filter for each channel to reduce the high frequencies where most of the objectionable noise exists and which further provides a blend of the two channels, which effectively cancels the portion of the difference of channels signal component in the two channels and thus reduces the noise in each audio channel originating as noise in the 23 to 53 kilohertz frequency band and being decoded along with the difference of channels signal. The amount of attenuation of the higher audio frequencies and the degree of blend of the two channels are simultaneously controllable in response to a control signal indicative of the noise level.
The apparatus and method of this receiver have been manufactured and sold on many vehicles by the assignee of this application and have been successfully received by the public. However, the audio processor requires a separate circuit chip added to the receiver circuitry between the stereo decoder and normal left and right channel audio amplifiers; and, within the chip, separate, identical circuits are required for each channel, along with the blend circuitry. This, of course, increases the cost of the receiver to the purchaser.
However, it has been discovered by the inventors of this invention that the similar high frequency roll-off of the higher audio frequencies and attenuation of the difference of channel signal may be obtained by the proper processing of the stereo composite signal before it is decoded into separate right and left channel signals. A low pass filter which has a high frequency roll-off characteristic in the audio frequency range and an extended constant attenuation characteristic in the difference of channels signal frequency range will produce a modified stereo composite signal which, when decoded, will produce the desired effect in the left and right channel audio signals. The circuitry for accomplishing this is considerably reduced in amount, and therefore in price and complexity, from that required for an audio processor that accomplishes the same result.